1. Field of the Invention
This invention relates to a multiple electron emission device having surface conduction type emission elements disposed two-dimensionally, and relates more particularly to a multiple electron emission device of this type in which unnecessary portions of electron emitting elements are made incapable of emitting electrons.
2. Description of the Prior Art
Conventional electron emission sources put to practical use as electron emitting devices are based on thermionic emission from a hot cathode or on field effect emission. A hot cathode type electron emission source emits electrons in such a manner that a large current is made to flow through a filament formed of tungsten or the like so that heat thereby generated gives electrons energy higher than the vacuum level. This type of electron emission source is being put into wide use, but none has yet been designed in the form of a two-dimensional array of electron emitting elements; one electron emission source of this type emits only one electron beam. Another example of practical thermal electron sources is represented by a linear electron source having a straight-line hot filament. This type of electron source, however, is not designed to have an elongated element due to thermal expansion of the filament. A field effect type electron emission source operating without heating emits electrons in such a manner that an intense electric field is applied from outside to a cathode chip having an acute tip. Electron emission characteristics of this electron source greatly depend upon the shape of the tip, and it is therefore difficult to prepare a plurality of electron emission sources of this type having the same characteristics. Therefore no multiple emission device using field effect electron emission sources has yet been put into practical use.
A type of device is known which, though simple in structure, is capable of emitting electrons. An example of this device is described on page 1290 of "Radio Engineering Electron Physics" 1965, volume 10, made public by Elinson et al.
The principle of this device is based on a phenomenon whereby electrons are emitted by making a current flow through a thin film formed on a substrate and having a small area in parallel with the surface of the film. Ordinarily, this device is called a surface conduction type emission device (in conformity with the description of the Thin Film Handbook).
Surface conduction emission devices other than the above example which utilize an SnO.sub.2 (Sb) film proposed by Elinson have also been proposed: one is based on a thin Au film (G. Dittmar: Thin Solid Films 9,317 (1972)), one is based on a thin ITO film (M. Hartwell and C. G. Fonstad: IEEE Trans. ED Conf. 519(1975)), and another is based on a thin carbon film (Hisashi Araki et al.: Vacuum, volume 26, No. 1, p. 22 (1983)).
FIG. 8 shows a typical arrangement of an element of these surface conduction emission devices. The element shown in FIG. 8 has electrodes 7 and 8 for electrical connection, a thin film 9 formed of an electron emitting material, a substrate 10, and an electron emitting portion 11.
To use the conventional surface conduction emission device, a process called foaming is initially performed before the operation of emitting electrons. That is, a voltage is applied between the electrodes 7 and 8 to energize the thin film 9 so that the thin film 9 is locally broken, deformed or changed in quality, thereby forming an electron emitting portion 11 having a high electric resistance.
This high-electric-resistance portion is a discontinuous film portion in which a crack having a width of 0.5 to 5 .mu.m is formed in part of the film 9, and a so-called islet structure is formed inside the crack. Ordinarily, in the islet structure, grains having a diameter of several tens of angstroms to arrange several microns exist in the form of a film on the substrate 10 in such a manner that the grains are spatially isolated from each other but are electrically continuous.
The conventional surface conduction emission element emits electrons from such grains when a voltage is applied between the electrodes 7 and 8 so that a current flows through the surface of the element.
The thermionic electron emission entails a problem of energy loss due to heating and, hence there is, a problem of heat radiation during emission of electrons, that makes arranging a plurality of thermionic electron emission elements on one substrate extremely difficult. A field effect electron emission device necessitates a process of acutely pointing the tip of the cathode chip until the radius thereof becomes several hundred angstroms. This process must include a number of steps including a remodeling step after the step of performing ordinary polishing, and is highly dependent on empirical factors. As a result, the possibility of dispersion of the elements during manufacture is high. It is very difficult to form a plurality of field effect electron emission elements having equal characteristics and to arrange these elements on one substrate. For this reason, no attempt to manufacture a practical electron emission device by providing a plurality of electron emission elements on one substrate has yet succeeded.
The conventional surface conduction emission device necessitates a foaming process during manufacture, and therefore entails the following drawbacks.
1) It is not possible to design the islet structure if electric power energization is adopted for foaming, and it is therefore difficult to improve the properties of the elements as well as to prevent dispersion of the same.
2) The islet structure has a short lifetime and is unstable, and the possibility of the element being broken by external electromagnetic noise is high.
3) Since the islet structure is formed by a foaming process, the degree of freedom of selecting the material for forming the islet structure is limited.
4) The shape of the element is limited because local concentration of heat is required in the foaming process.
5) The substrate tends to be broken by local concentration of heat.
Surface conduction emission devices have not been actively utilized by industry, even though they offer the advantage of having a simple structure.